This application is based on Japanese Patent Application No. 2001-240461 filed on Aug. 8, 2001, the disclosure of which is incorporated herein by reference.
The present invention relates to a starter having a one-way clutch that slides on an output shaft along helical splines for transmitting rotation of the output shaft to a pinion.
In a starter disclosed in JP-U-1-130074, rotation of an armature is transmitted to a pinion through a one-way clutch. The one-way clutch is provided with a cylindrical-shaped barrel. The barrel is engaged with an outer circumference of an output shaft extending from a rotary shaft of the armature through helical splines and is slidable thereon in an axial direction.
In general, helical splines of an output shaft are not formed by cutting, but formed by rack rolling to improve productivity. Also, helical splines of a barrel of a one-way clutch are generally formed by broaching or emboss pressing to improve productivity. However, manufacturing quality of the splines is likely to be lessened as compared with that of helical splines formed by cutting.
If the manufacturing quality of the splines is low, slidability between the splines becomes worse. For example, if loads are applied on the splines due to shock when a pinion meshes with a ring gear, splines are likely to adhere (bite) with each other.
The present invention is made in view of the above problem, and it is an object of the present invention to provide a starter having enhanced productivity.
It is another object of the present invention to provide a starter in which slidability of spline portions is improved.
It is further another object of the present invention to provide a method of chemical treatment for forming a chemical conversion coating to enhance slidability of helical spline portions of a starter.
According to the present invention, a starter has an output shaft for outputting a rotation force of a motor, and a one-way clutch provided with a barrel fitted on the output shaft for transmitting rotation of the output shaft to a pinion. The output shaft and the barrel include spline portions having helical splines and engage with each other. Chemical conversion coating is formed on at least one of the spline portion of the output shaft and the spline portion of the barrel. Microscopic asperities are formed on the surface of the chemical conversion coating.
According to this structure, grease applied on the spline portions is held by the microscopic asperities of the chemical conversion coating. Therefore, oil-holding capacity is improved and durability against wear is increased. Further, with the chemical conversion coating, the spline portions are prevented from rusting even under fatigue environments of the water and dirt. Accordingly, slidability can be ensured between the output shaft and the barrel.
Alternatively, microscopic asperities can be directly formed on at least one of the spline portion of the output shaft and the spline portion of the barrel. With this, grease applied on the spline portions is held by microscopic asperities of the spline surfaces. Therefore, oil-holding capacity can be improved and durability against wear can be increased. Further, because oil film breakage can be decreased with the improvement of the oil-holding capacity, it is possible to minimize an area where a base material of the output shaft press-contacts a base material of the barrel, thereby ensuring slidability.
A method of chemical treatment for forming the chemical conversion coating on the spline surfaces includes steps of washing the spline portion for removing dirt, chemical treating the spline portion by immersing in a chemical treatment solution and removing the chemical treatment solution from the spline portion by washing. With this process, the chemical conversion coating can be properly formed on the spline surfaces.